1. Field of the Invention
The present invention relates to a substrate having a power line or ground line, and to a ceramic package having the same.
2. Description of the Prior Art
With increasing demand for more compact and higher performance electronic devices, the high density mounting or packaging of active and passive elements and the like is being recognized as the key problem to be solved. To cope with the problems that are encountered in mounting or packaging the elements at a still higher density, there is a tendency of using multilayered substrates for mounting thereon the active and passive elements, or of employing multilayered packages for packaging semiconductor chips. The multilayered substrates include a multilayered printed wiring board and various types of multilayered wiring substrates specially used for mounting hybrid ICs and multi chip modules. Examples of the packages for packaging semiconductor chips, which are composed of a plurality of laminated low-temperature cofired ceramic sheets, include a Pin Grid Array (hereinafter abbreviated as "PGA") package, a Land Grid Array (LGA) package, and a Quad Flat Package (QFP). Any of the multilayered printed wiring boards and the multilayered wiring substrates enumerated above is referred to hereinafter simply by the generic term "substrate", and any of the packages above is sometimes referred to hereinafter simply by the generic term "ceramic package".
In general, a ceramic multilayered wiring substrate or a ceramic package comprises, as shown in FIGS. 1A-C, an insulating layer 100 having a signal line 102 on one side thereof, and a power line or ground line 104 on the other side thereof. Another insulating layer 100A located under the power line or ground line 104 is also shown in the figure. Though not shown in the figure, another insulator layer is provided on the upper side of the signal line 102.
A schematically drawn partial plane view showing the relation between the signal line 102 and the power line or ground line 104 is given in FIG. 1 (A). The insulating layers 100 and 100A, however, are excluded from the figure. FIG. 1 (B) is a schematically drawn partial cross section view along the line B--B, and FIG. 1 (C) is a schematic view of the power line or ground line 104 patterned into a mesh. The region defined inside the squares in FIG. 1 (C) has no power lines or ground lines formed therein.
An LGA package comprising a plurality of laminated low-temperature cofired ceramic sheets can be fabricated, for instance, by a process comprising: processing a green sheet containing borosilicate glass and the like as the principal component into a sheet of a predetermined size; perforating the sheet by punching and the like to provide holes for use as through holes or via holes; burying the thus perforated holes in the green sheet with a metallic paste containing gold, silver, copper, etc., as the principal component; screen printing the metallic paste on the green sheet to provide signal lines 102 and power lines or ground lines 104 on the surfaces of the green sheet; and forming, for example, through holes or via holes for electrically connecting the signal lines 102 and the power lines or ground lines 104, land portions as a connecting portion with the external circuits, and connection terminals for the semiconductor chips (such as wire bonding portions). Then, a predetermined number of such green sheets are fabricated, laminated, and sintered simultaneously in a temperature range of from about 800.degree. to 1,000.degree. C. Thus is obtained a complete LGA package comprising low temperature cofired ceramic sheets, having a signal line 102 on one side of the insulating layer and a power line or ground line 104 on the other side of each of the laminated ceramic sheets.
In the process described above, gas evolves during the sintering or the cofiring. If the power line or ground line should be formed over the entire surface of the low temperature cofired ceramic sheet instead of providing it in a meshed pattern, the gas evolved from the green sheet would remain undissipated. The gas thus evolved would then be trapped between the green sheets to cause inter-layer separation. Furthermore, the evolution of gas during cofiring impairs the dimensional stability of the green sheet. Thus, in case of using a green sheet which evolves gas, in general, the power line or ground line is formed into a meshed pattern.
A multilayered printed circuit board can be fabricated, for instance, by etching a first copper foil of a copper-clad glass epoxy laminate or a flexible copper-clad polyimide laminate (hereinafter collectively referred to simply as "the copper-clad laminate") to form a power line or ground line on the first copper-clad laminate to give the inner layer, laminating this inner copper-clad laminate with a second copper-clad laminate for the outer layer using a hot press, and copper plating and etching the second copper-clad laminate provided as the outer layer to form a circuit inclusive of the signal line and the like.
Also in multilayered printed circuit boards, there are cases of forming a mesh-patterned power line or ground line on the copper-clad laminate for the inner layer to prevent warping or twisting from occurring on a substrate, e.g., a copper-clad laminate, and to improve the dimensional stability of the substrate.
A certain type of multilayered wiring substrate for use in hybrid ICs and multi chip modules comprises a base made of silicon or ceramics, formed thereon a plurality of signal lines and power lines or ground lines insulated by a polyimide resin insulator layer. Bare chips, for example, are mounted on these types multilayered wiring substrates.
In these types of multilayered wiring substrates for hybrid ICs and multi chip modules, the power lines or ground lines are sometimes formed in mesh patterns. In this manner, the gas generated from polyimide during thermosetting the polyimide resin can be discharged to prevent inter layer separation from occurring, and the dimensional stability of the substrate can be improved.
Referring to FIGS. 1A-C again, when a mesh-patterned power line or ground line 104 is formed, the power line or ground line 104 not always is found under the signal line 102 (see FIG. 1 (B)). More specifically, the signal line 102 may or may not run over the power line or ground line 104. The high frequency characteristics, the characteristic impedance in particular, changes depending on the presence of the power line or ground line 104. This causes a problem of impedance discontinuity in the transmission line.
Accordingly, an object of the present invention is to provide a substrate and a ceramic package having a power line or ground line which less influences the high frequency characteristics of the signal line.